It is well known to use a low voltage (12 volt) starter motor to start an internal combustion engine even when the engine is at a very low temperature such as for example minus 30° C. It is further known to provide an engine with a high voltage (for example 48 volt) belt integrated starter generator (BISG) that is driveably connected to a crankshaft of the engine by a drive belt and is able to be either driven by the engine to generate electrical power or drive the engine either to assist with power output or for starting the engine.
Normally the BISG is only used when the temperature of the engine is above a relatively warm temperature corresponding to a low end of a temperature range of normal engine operation such as for example 60° C. because a conventional BISG will normally have insufficient torque to start the engine at lower temperatures.
It is further known that the fuel saving ability of a vehicle is restricted or limited by the need to first light-off any exhaust aftertreatment devices. It is therefore desirable, particularly when the engine is cold, to apply a load to the engine after it has started in order to increase the temperature of the exhaust gas and thereby reduce the time taken to light-off any attached exhaust aftertreatment devices.
The BISG can be usefully employed to apply such a heating load but is often not able to do so because the current state of charge (SOC) of the associated high voltage battery will not permit the BISG to be run as a generator.
One advantage of using the BISG over a larger temperature range to start the engine is therefore that the discharging effect of using the BISG to start the engine can beneficially be used to reduce the state of charge of the high voltage battery thereby allowing the BISG to be subsequently operated as a generator after the engine has started to apply a load to the engine and speed up light-off of the exhaust aftertreatment devices.
A further advantage of using the BISG over a larger temperature range to start the engine in the case of a mild hybrid vehicle is that stop-start operation does not have to be restricted until the engine temperature has reached the low end of the temperature range of normal engine operation (60 to 120° C.) but can be used whenever BISG starting is possible thereby increasing the opportunities for stop-start operation.
Therefore it is desirable to use the BISG whenever possible to start the engine.
GB Patent application 2,486,708 discloses a method and system in which a temperature limit is used to determine whether to use the BISG to start the engine. The temperature limit being set primarily on whether the NVH performance of the motor vehicle will be adversely affected by use of the BISG to start the engine compared to the use of a starter motor. However, the temperature limit is set relatively high because it is possible to breach this limit when queuing in traffic in cold ambient conditions.